Hydrocarbons in insects

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

Kunst, 1997) and yeast (Oh et al., 1997). The role of fatty acyl-CoA elongation in regulating the chain length of hydrocarbons in insect pheromone biosynthesis is discussed below (8.4.6). 

Chu A. J. and Blomquist G. J. (1980) Biosynthesis of hydrocarbons in insects succinate is a precursor of the methyl branched alkanes. Arch. Biochem. Biophys. 201, 304-312. 

Figure 1. Origin of the methylmalonyl-CoA used for the biosynthesis of methyl-branched hydrocarbons in insects.

It is well established that propionate can be utilized for the methyl branch unit in methyl branched hydrocarbons in insects (4-7), and recent data have shown that the methyl branches are inserted early during chajij elongation rather than toward the end of the process (13,16). C-NMR analysis demonstrated that propionates labeled with C in either the 1, 2 or 3 positions are incorporated into the methyl branched alkanes of insect cuticular lipids. C-3 of propionate becomes the branching methyl carbon, C-2 becomes the tertiary carbon and C-l the carbon adjacent to the tertiary carbon (13,16,17) in these methyl branched hydrocarbons. 

Many of the hydrocarbons in insect lipids are also of rather high molecular weight and it is not unusual to find reports of lipids containing hydrocarbons up to C]5o, not necessarily as homologous series but simply as individual compounds. The surface lipids of the field cricket (Nemobius fasciatus) are 100% hydrocarbons (Nelson Blomquist 1995). Long-chain branched alkanes have been reported in the grasshopper (Schistocerca vaga), where the... 

Insect cuticular hydrocarbons are commonly identified on the basis of retention indices (Nelson Blomquist, 1995). Pomonis et al. (1989) determined the Kovats retention indices of monomethyl-jrentacosanes, some internally branched dimethylalkanes and 2,x-dimethylheptacosanes on a cross-linked methyl silicone fused silica capillary column (Hewlett-Packard). Carlson et al. (1998) described a protocol for the identification of methyl-branched hydrocarbons in insect cuticular waxes. In this protocol, programmed-temperature retention indices are assigned to peaks, then the patterns in GC peaks that probably contain homologues are marked to assist subsequent GC-MS interpretation. The authors also included data from the literature covering most of the insect methylalkanes. 

Many short chain fatty acids, aldehydes, alcohols, esters, ketones and hydrocarbons are produced by metabolism of fatty acids (Cie-Cis). These compounds are common in essential oils and are also found in insects. 

It has been suggested that the ethers, compounds unique to spiders, may provide reliable signals for pattern recognition and species determination. In contrast, a pattern of hydrocarbons, as used in several insect species, might be susceptible to contamination from cuticular hydrocarbons from insect prey remnants, which might alter the blends produced by the spiders and deposited on the webs (Schulz, 1997a, 1999). 

Blomquist, G. J., Tillman-Wall, J. A., Guo, L., Quilici, D. R., Gu, P. and Schal, C. (1993). Hydrocarbon and hydrocarbon-derived sex pheromones in insects biochemistry and endocrine regulation. In Insect Lipids Chemistry, Biochemistry and Biology, eds. 

Many chlorinated hydrocarbon alternatives to DD T have been developed. One of the earliest substitutes was methoxychlor, shown in Figure 15.18. This compound has a much lower toxicity in most animals and, unlike DDT, is not readily stored in animal fat. Look carefully at the structures of methoxychlor and DDT, and you ll see that they are identical except that methoxychlor has two ether groups where DDT has two chlorine atoms. Because the structures are nearly identical, they have nearly the same level of toxicity in insects. In higher... 

Subchev M. and Jurenka R. A. (2001) Sex pheromone levels in pheromone glands and identification of the pheromone and hydrocarbons in the hemolymph of the moth Scoliopteryx libatrix L. (Lepidoptera Noctuidae). Arch. Insect Biochem. Physiol. 47, 35 13. 

Young H. P., Bachmann J. A. S., Sevala V. and Schal C. (1999) Site of synthesis, tissue distribution, and lipophorin transport of hydrocarbons in Blattella germanica (L.) nymphs. J. Insect Physiol. 45, 305-315. 

Blomquist, G. J., Tillman J. A., Mpuru S. and Seybold, S. J. (1998) The cuticle and cuticular hydrocarbons of insects structure, function and biochemistry. In Pheromone Communication in Social Insects, eds R. K. Vander Meer, M. Breed, M. Winston and C. Espelie pp. 34-54. Westview Press, Boulder, CO. 

Chase J., Jurenka R. J., Schal C., Halamkar P. P. and Blomquist G. J. (1990) Biosynthesis of methyl branched hydrocarbons in the German cockroach Blattella germanica (L.) (Qrthoptera, Blattellidae). Insect Biochem. 20, 149-156. 

In insects, especially Diptera, several pioneer studies reviewed by Blomquist et al. (1987) established that long chain hydrocarbons, some of which play a pheromone role, were derived from very long chain fatty acids by reduction and decarboxylation. Thus, pheromone biosynthesis shares steps with those leading to basic lipid molecules and also with those of the well-known pheromones of Lepidoptera (Roelofs and Wolf, 1988). All often display several double bonds located in various positions while the volatile butterfly compounds bear functional groups (acetate, aldehyde or alcohol) and aliphatic chains with 12-16 carbons. Contact pheromones of flies have much longer chains (21C-39C) (Pennanec h et al., 1991). 

El Messoussi S., Wicker C., Arienti M., Carlson D. A. and Jallon J. M. (1994) Hydrocarbons in species recognition in insects. In Identification and Characterization of Pest Organisms, ed. D. Hawskworth, pp. 277-287. CABI Press, London. 

Oenocytes have been shown to biosynthesize hydrocarbons in several insect species in which the oenocytes are within the hemocoel and could be readily separated from other tissues (Diehl, 1973, 1975 Romer, 1980, 1991). These cells are characteristically very large, among the largest somatic cells in insects, and are rich in mitochondria and SER, as are steroidal cells in mammalian systems, suggesting participation in lipid synthesis (Rinterknecht and Matz, 1983). In the German and American cockroaches, however, the oenocytes are... 

Cuvillier-Hot V., Cobb M., Malosse C. and Peeters C. (2001) Sex, age and ovarian activity affect cuticular hydrocarbons in Diacamma ceylonese, a queenless ant. J. Insect Physiol. 47, 485 -93. 

The long-chain hydrocarbons of insects play central roles in the waterproofing of the insect cuticle and function extensively in chemical communication where relatively non-volatile chemicals are required. The recognition of the critical roles that hydrocarbons serve as sex pheromones, kairomones, species and gender recognition cues, nestmate recognition, dominance and fertility cues, chemical mimicry, primer pheromones and task-specific cues has resulted in an explosion of new information in the past several decades, and, indeed, served as the impetus for this book. 

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

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